Copending application Ser. No. 598,892 filed April 1984 and entitled "Real-Time Text-To-Speech Conversion System" discloses a text-to-speech conversion system in which digitized waveforms representing constituents of speech are stored in a random access memory, and are assembled into phonemes and transitions under the control of a program which reads computer-formatted text and determines therefrom which stored waveforms are to be used, and in what manner, to create spoken words corresponding to the text.
A major problem in using all-software text-to-speech conversion programs in personal computers is the inadequacy of available memory for high-quality speech production. Consequently, it is necessary to compact the stored waveforms so that a great deal of waveform data can be stored in a small amount of random access memory.
In addition to using the compaction-methods described in Ser. No. 598,892, it has previously been proposed to compress digitized waveforms by an "optimal delta" compression technique illustrated in U.S. Pat. No. 4,617,645. This technique is reasonably efficient, but it introduces a slight amount of distortion into the re-created analog waveform.
Although these methods were satisfactory in early text-to-speech conversion products, the continuing need for ever more natural-sounding artificial speech has made it necessary to develop more powerful compression methods in order not only to store more digitized waveforms within the limits of available memory, but also to reduce the amount of program memory involved in assembling the stored waveforms to produce speech.